Variable magnification cathode ray tube



R. H. COMPTON VARIABLE MAGNIFICATION CATHODE RAY TUBE Filed Aprn 29, 1968 zoiumwq March 17,v 1970 e TM ATTORNEYS INVENTOE @afer/z CoMro/v ZOCOmJmm filial/Afl! United States Patent m U.S. 'Cl. 315-31 10 Claims ABSTRACT OF THE DISCLOSURE A display device is described comprising a shaped beam tube which includes a plurality of lens elements forming two electron lenses. The focusing effect of each lens is controllable in response to a variable electric signal to vary image size while maintaining image sharpness.

This invention relates to display devices and, more particularly, to an improved cathode ray tube of the character generating type for displaying character information and wherein character size is variable.

-In character generating tubes of the shaped beam type, one or more electron beams are shaped as they pass from an electron gun to a target such that the resulting cross section of each shaped beam is of predetermined conguration. The area of the target energized by impingement of each beam thereon therefore corresponds in shape to the resulting cross section of the beam. The desired beam cross section may be attained by passing the beam through a shaping aperture in an electron opaque plate.

After passing through a shaped aperture, the cross section of the electron beam in a character generating tube of the shaped beam type will conform to a desired character shape. The beam is then directed to impinge upon a target screen, such as a phosphor coating screen, and thereby energize or illuminate an area of the screen corresponding in shape to the shape of the aperture through which the beam passed. The screen may be constructed for such purposes as to provide a visible display for direct observation or for photographing onto lm, such as microlm. The display may also be utilized in connection with an electrostatic printer or other type of printer.

In order to produce Va legible and sharp image on the target display screen, character generating tubes generally include an alactron lens arrangement for magnifying the character defined by the character shaped aperture and producing a sharp image on the target screen. Under certain circumstances, it may be desirable that the image, delining a character or other iigure, be variable in size on the screen of the tube. Naturally, it is desirable that such variation in size not have a deleterious effect on the quality of the image (i.e., that the image be sharp and of satisfactory brightness).

Accordingly, it is an object of this invention to provide an improved display device.

Another object of the invention is to provide an improved cathode ray tube for displaying character information on a target screen.

A further object of the invention is to provide a display device utilizing a character generating tube and in which character size is variable.

It is another object of the invention to provide a display device utilizing a character generating tube wherein character size is variable while maintaining a high quality image.

Other objects of the invention will become apparent to those skilled in the art from the following description 3,501,673 Patented Mar. 17, 1970 taken in connection with the accompanying drawings wherein: l

FIGURE 1 is a lschematic sectional view of a display device constructed in accordance with the invention;

FIGURE 2. is a graphical representation illustrating, generally, the operation of a portion of the device of FIGURE 1; and

FIGURE 3 is a schematic view of a portion of a display device illustrating an alternative embodiment of the invention.

Very generally, the display device of the invention includes a cathode ray tube 11 having a target screen 12 and an electron beam source 13. Means 14, 15 and 16 are included in the cathode ray tube for accelerating the electron beam to impinge on the target screen. Means 17 are provided for forming the electron beam into a desired cross section. This results in the production of a correspondingly formed image on the target screen. Electron lens means 19 including hollow conductive lens elements 21 and 22 are disposed between the forming means and the target screen for producing a sharp image of the beam cross section on the target screen. The lens means dene at least two electron lenses each having an effective focal length which is variable in response to an electric signal applied to the lens elements. Means 23 and 24 are provided for selectively varying the electric signals applied to the lens elements 21 and 22 to vary the image size on the target screen while maintaining image sharpness.

Referring now more particularly to FIGURE 1, the schematic diagram of the display device illustrates -a character generating cathode ray tube 11 of the shaped beam aperture selection type. The glass or ceramic envelope of the tube is indicated by the dotted line 26 which is formed with a bell-shaped section joined to an elongated necked section. `One end of the bell-shaped section is closed by a suitable glass face plate, not illustrated. The target lscreen 12 is supported on the inner surface of the fresh plate. Als is common in the art, the target screen may be a coating of -a suitable electron sensitive light emitting material so that a visual display will be produced by impingement of electrons on the coating.

The electron beam originates at a cylindrical cathode 13 which surrounds a heater 27. The cathode may be comprised of nickel coated on its end surface with barium oxide or some other similar material which produces free electrons at elevated temperatures. A suitable electrical connection is made to the heater 27 for passing a heating current therethrough, and the cathode itself is maintained at a suitable potential such as ground. The free electrons emitted by the cathode are directed into a beam 30 by accelerating elements 14 and 16 so that the beam will impinge upon the target screen 12. Between the element 1'6 and the screen 12, the beam is very small and therefore only the axis of the beam 30` is indicated, Whereas elsewhere, the envelope of the beam is also indicated. The accelerating elements 14 and 16 are maintained at a positive potential with respect to the cathode from an accelerating potential source 28. The target screen 12 is maintained also at a substantial positive potential from an anode potential source 29. Suitable circuitry (not illustrated) for providing and regulating such potentials at the respective sources 28 and 29 may be provided. In addition, a grid element 74 may be provided adjacent the cathode 13 in order to shut olf and turn on the electron beam, and to collimate the beam emitted by the cathode 13.

In order to shape the cross section of the beam into the form of a desired character, the matrix plate 17 is provided. The matrix plate is `supported in the element 15 and defines a plurality of apertures 18. Each of the apertures is shaped in the form of a desired character and the apertures are arranged in a matrix. Element and the aperture plate 17 are maintained sufficiently positive with respect to the cathode by the source 71 that electrons are drawn from the cathode region and projected through the apertures in plate 17. The electron beam passing through the apertures 18 thus is segmented into a bundle of individually shaped beams by the matrix plate. For simplicity, the bundle of shaped beams is indicated as the single beam 30 in the drawings.

A focusing electrode 31, connected to a variable potential source 25, in conjunction with elements 15 and 14, forms an electron lens which converges the beam 30 and produces an electron image of the aperture plate 17 in the plane of a plate 33.

The beam or bundle of shaped beams 30 is converged to a minimum cross section at a point 32 and then diverges to its original size or (preferably) a somewhat greater size in the plane of the aperture plate 33. The plate 33 is supported in the accelerating element 16 and defines an aperture 34. The aperture 34 is large enough to permit only one of the shaped beams in the bundle 30 to pass therethrough without affecting its shape. The other shaped beams are blocked by the plate 33.

Between the matrix plate 17 and the aperture plate 33, the entire bundle 30 is deflected to select the particular shaped beam in the bundle which is to be passed through the aperture 34. Deflection is accomplished by suitable pairs of horizontal and vertical deflection plates 35, 36 and 37, coupled to a selection circuit 38. Thus, according to the operation of the selection circuit, which may be controlled by a computer, not shown, an image of a selected aperture in the matrix plate 17 will appear at aperture 34.

The matrix plate 17 comprises an electron opaque plate in which the previously mentioned plurality of apertures 18 are formed. The apertures correspond to the shape of desired display portions, such as alphanumeric characters or graphical line segments, and shape the beam as it passes therethrough. Deflection of the beam 30 may thereafter be accomplished in cooperation with the aperture 34 to select the shaped beam from a desired aperture 18 and thereby select the shape of a particular image to be displayed upon the target screen 12.

After passing through the aperture 34, the shaped electron beam 30 passes through the lens means 19 (comprising the two lens elements 21 and 22) in which it is focused so that a sharp image is produced on the target screen 12. As explained below, the magnification provided by the lens means 19 is variable to enable variation in character size while retaining a sharp image.

The position of the image on the target screen is controlled by a beam deflection yoke 39 surrounding the neck of the cathode ray tube envelope 26 adjacent the bell-shaped portion thereof. A suitable deflection circuit 41 controls the yoke 39 in order to produce the desired beam deflection, such as is indicated by way of example in FIGURE l. A dag section 40 is provided as a coating of colloidal graphite or other suitable material on the inner surface of the tube envelope 26 in the region shown, and is maintained at the high positive potential of the screen 12, with respect to the cathode, to accelerate the electrons.

In the embodiment illustrated in FIGURE l, the lens elements 21 and 22 are not true cylinders. Rather, the ends of the elements 21 and 22 which are toward the cathode are of reduced diameter. For purposes of approximate calculations, however, both lens elements 21 and 22 may be considered cylindrical. The effective diameter of the lens element 21 in the region of the space between it and the accelerating electrode 16 is the inside diameter of its small end, while the effective diameter of the lens element 21 in the region of the gap between it and the lens element 22 is the inner diameter of its larger end. The effective diameter of the lens element 22 at the region of the gap between it and the lens element 21 is the inner diameter of its small end. The effective diameter of the lens element 22 in the region between it and the dag section 40 is the inner diameter of its larger end.

In the embodiment illustrated in FIGURE l, there are three effective gaps or potential discontinuities along the axis of the tube between the plate 33 and screen 12, all of which may contribute to the focusing effect of the lens means 19. These gaps are between the accelerating electrode 16 and the lens element 21, between the lens element 21 and the lens element 22, and between the lens element 22 and the dag `section 40. For a given set of dimensions onthe various elements, the maximum and minimum available magnifications may be approximated by using the formula where M is the magnification, k is a constant determined by the ratio of the diameter of the cylinders at each gap, and Q and P are the image and object distances, respectively, as measured from the appropriate gap under consideration.

By way of specific example, a satisfactory system may be constructed utilizing the following approximate dimensions of the lens components:

Inside diameter of accelerating electrode 16-0.74 inch Inside diameter of small end of lens element 21-0.49

inch

Inside diameter of large end of lens element 21-0.75

inch

Inside diameter of small end of lens element 22-0.49

inch

Inside diameter of large end of lens element 22--0.75

inch

Object distance, cathode end of lens element 21 to aperture 34-03 inch Object distance, cathode end of lens element 22 to aperture 34-0.8 inch Object distance, target end of lens element 22 to aperture 34-l.5 inch Image distance, cathode end of lens element 21 to screen- 8.5 inches Image distance, cathode end of lens element 22 to target- 8.0 inches Image distance, target end of lens element 22 to target- 7.3 inches k ratio for accelerating electrode 16-lens element 21 gap-0.82 k ratio for lens element 21-lens element 22 gap-0.82 k ratio for the lens element 22-dag section 16 gap-0.75

If all focusing were effected by the accelerating electrode 16-lens element 21 gap, the maximum magnification available is:

If all focusing were effected at the lens element 22-dag section 16 gap, the minimum magnification would be:

Mmin =0.75 `f3=3.6

The maximum available zoom range is therefore approximately 6 to l.

In a tube having the foregoing approximate dimensions for the various elements, satisfactory results may be obtained by operating the accelerating electrode 16 at +3,000 volts with respect to the cathode, the dag section 40 at +18,000 volts with respect to the cathode, the lens element 21 at approximately +3,000 volts with respect to the cathode, and the lens element 22 at approximately $2,000 volts with respect to the cathode. A character height of 0.022 will be obtained with a satisfactorily sharp image at the target screen. If somewhat larger characters are desired, the lens element 21 may be operated at approximately 1,000 volts with respect to the cathode and the lens element 22 may be operated at approximately 3,000 volts with respect to the cathode. lSuch voltages provide a satisfactorily imaged character height of approximately 0.028 inch. Exact required voltages will vary somewhat from tube to tube due to variation in size of the various elements within manufacturing tolerances.

Various other combinations Vof voltages on the lens elements 21 and 22 may be utilized to provide a variety of sharply imaged character sizes. Such voltages, for satisfactory imaging, may be computed by means of raytracing techniques. The fields in the illustrated arrangement are of such complexity, however, that ray tracing techniques for voltage computation would, as a practical matter, require use of a digital computer. Reasonable practical estimates may be made by observing the following rules:

(a) In a simple lens increasing the voltage ratio reduces the image distance and the magnification if the object distance is fixed, i.e., it increases the strength of the lens.

(b) In a simple lens increasing the object distance decreases the image distance and the magnification, if the voltage ratio is fixed.

(c) In a compound lens the combination is a stronger lens than either of the simple lenses alone, and increasing the strength of either simple lens increases the strength of the combination.

(d) In a compound lens, an increase in the strength of a simple lens near one end of the structure moves the effective center of the compound lens toward that end.

As a practical matter, the potentials required on the lens elements 21 and 22, to achieve sharp imaging at a given character height, are usually best determined empirically. One procedure for doing this is as follows:

(l) The tube is located in a darkened room, or the tube screen is viewed through a hood which excludes room light. The electron beam in the tube is turned on and is deflected to a position such that no beam is passing through the aperture 34. Under these conditions, a shadow of the aperture 34 may be seen on the screen caused by stray electrons which pass through the aperture and are then accelerated to the phosphor screen. Accordingly, the voltages on the lens elements may be adjusted to produce a sharply focused image of the aperture 34 itself, rather than a shaped electron beam within the aperture.

(2) The lens element 21 is arbitrarily set at a potential which is convenient and which is qualitatively selected for a desired magnification. The general rule that a gap having a large potential difference is a strong lens and a gap having a small potential difference is a weak lens may generally be followed in this connection. If a high magnification is desired, with consequently relatively larger characters, the voltage on the lens element 21 may be set at a potential which is relatively far from the potential of the accelerating electrode 16. If`only small magnification is desired, the lens element 21 potential may be set relatively near the potential of the accelerating electrode 16. The potential on the lens element 22 may then be adjusted until the outline of the aperture is sharply imaged.

(3) A character shaped beam is then selected and passed through the aperture 34. The character height on the target screen is then measured. If this height is too large, the potential on the lens element 21 is changed to a value nearer that of the accelerating electrode 16, and the lens element 22 potential is again adjusted to produce sharp focus of the aperture.

(4) The character height is again measured, and if too small, the potential on the lens element 21 is again changed to a value farther from that on the accelerating electrode 16. The potential on the lens element 22 is again adjusted for sharp focus. It may be noted that lens action will occur at the gap between the accelerating electrode 16 and the lens element 21 as long as there is a difference of potential at the gap, and this difference of potential may be relatively positive or relatively negative.

Since the aperture plate 33 is supported by the accelerating electrodes 16, it is convenient, from a practical standpoint, to maintain the potentials of these two elements the same. Moreover, this potential is held fixed with respect to the cathode, since any variation of this potential affects the adjustment of the other elements between the cathode and the aperture plate and thus causes interactions between the adjustments for the desired character selection and the magnification.

The dag section 40 is also maintained at a xed potential with respect to the cathode so that the deflection sensitivity of the deflection yoke 39 is fixed. The deflection sensitivity (inches per ampere) of the yoke is proportional to the square root of the potential through which the electrons are accelerated prior to deflection. Thus, a fixed dag section potential avoids interaction between the adjustment of character size through adjustment of the potentials on the lens elements 21 and 22, and the adjustment of total beam deflection in accordance with the gain of the yoke driving amplifier in the control circuitry. The potentials on the lens elements 21 and 22 may, however, be conveniently varied. The possibility of character size adjustment is of significant advantage in commercial production even when the tube is to be operated at only one character size. Such tubes are readily adjusted for the specified character size and no tubes need be discarded because of improper character size, even though manufacturing tolerances permit the dimensions of individual electron guns to vary.

In order to provide for variation of potential on the lens elements 21 and 22 to vary image size while maintaining sharpness, suitable circuitry is provided. In the ernbodiment illustrated in FIGURE l, the lens element 21 is connected through a switch 23 to one of a plurality of different focus potentiometers 45, all energized by a focus potential source 42. Similarly, the lens element 22 is connected through a switch 24 to one of a plurality of different focus potentiometers all energized by a focus potential source 43. It may be seen that by operating the switches 23 and 24, the potential upon the respective lens elements 21 and 22 may be varied depending upon the potentiometers selected. The potentials of the various potentiometers are determined in accordance with the foregoing described empirical techniques to provide for variation in character size while maintaining sharpness. The switches 23 and 24 are ganged and the terminals are so connected that only voltage combinations are selected that provide a sharp image.

The lens arrangement and focusing circuits of the display device of the invention are somewhat similar to a varifocal or zoom lens optical system. However, rather than changing the position of the lenses relative to each other to change effective focal length, as is usually the case in a varifocal lens optical system, the focusing effect of each of the lenses themselves is altered in a predetermined manner. Where the lens at the gap between elements 16 and 21 is of sufficient strength to focus the aperture plane onto thescreen plane alone, and the lenses at the gap between elements 21 and 22 and the gap between the element 22 and the dag coating gap are relatively weak, the maximum magnification is produced. When the lens at the element 22-dag gap is strong, and the lenses at the other gaps are weak, the smallest magnification is produced. If the only lens with strong focusing effect is at the element 21-22 gap, intermediate magnification results. Any magnification between the maximum and minimum may be obtained by using combinations of lenses of moderate or intermediate strength. For example if the lens at the element 16-21 gap is moderately strong, the lens at the 21-22 gap is also moderately strong, and the lens at the 22-dag gap is weak, the magnification will be large but less than the maximum. Strong lenses are achieved when the ratio of potentials across a gap is large.

To summarize the operation of the device, operation of the lenses 21 and 22 in FIGURE 1 may be thought of as a compound electron lens for which the effective position of the lens is shifted along the tube axis by appropriate voltage variation. Since the magnification is proportional to the quotient of the lens to image (screen) distance divided by the lens to object (aperture) distance, voltage variation may produce an effective decrease in the numerator and increase in the denominator, or vice versa.

A graphical approximation of the manner in which the lens arrangement 19 achieves variable magnification is illustrated in FIGURE 2. The object plane, which is in the plane of the aperture plate 33 is indicated by the line 44. The image plane, which is the target screen 12, iS indicated by the line 46. The general focusing region of the lens at the 1621 gap is indicated by the line 47, and the general focusing region of the lens at the element 22dag gap is indicated by the line 48. The effective position of the compound lens is variable between the lines 47 and 48 along the axis. Naturally, FIGURE 2 is a simplification and does not illustrate the detiection of the beam itself but only approximates the deflection of electrons in the beam with respect to the axis. The aperture size is indicated by the arrow 49 in the object plane 44. For a relatively lower degree of magnification, the electron trajectories corresponding to analogous light rays are indicated in solid lines. It may be seen that such rays or trajectories converge at the tip of the arrow 51, this tip being substantially farther from the axis 52 than the tips of the arrow 49 and representing a substantial order of magnification. The effective position of the compound lens is at point A.

The dotted lines in FIGURE 2 represent the result when the potentials of the lens elements 21 and 22 are varied to produce a relatively higher order of magnification. The effective position of the compound lens is at the point B. It will be seen that although the arrow 53 is substantially farther from the axis 52 than the arrow 51, representing a correspondingly higher order of magnification, the electron beam trajectories or rays corresponding to the tip of the arrow 49 converge at the tip of the arrow 53, still in the image plane. This indicates that the image remains sharply defined despite the increased order of magnification.

Referring now to FIGURE 3, an alternative construction for the lens arrangement 19 is illustrated. The alternative lens arrangement 54 is positioned immediately after the accelerating element 16 and comprises a pair of hollow conductive lens elements 56 and 57 separated by a further accelerating element 58. The lens elements are connected via the switches 23 and 24 and the focus potentiometers 45 to the focus potential sources 42 and 43. The accelerating element 58 is connected to the accelerating potential 28 along with the accelerating element 16. In this embodiment of the invention, the operation is the same as that illustrated in FIGURE l, but, due to the interposition of an accelerating element 58 between the lens elements 56 and 57, the lens arrangement S4 extends over a greater length of the tube axis and a broader range of character size variation is attainable. The voltages applied to the respective lenses S6 and 57 in the embodiment of FIGURE 3 may be selected empirically in accordance with the same factors mentioned in connection with FIGURE 1.

The embodiment of FIGURE l may also be operated in an alternative mode, wherein the potentials applied to the electrodes are substantially different from those described hereinbefore, and the magnification is greater. The alternative mode of the FIGURE 1 embodiment may be referred to as the cascade lens mode, and is analogous to an optical system using a sequence of relay lenses to form successive new images. In the cascade lens mode, a large potential difference is applied across the gap between the accelerating element 16 and the lens element 21. The strong focusing effect of the element 16-21 gap produces a crossover or plane of minimum cross section within the element 21 and an image of the aperture plane is formed in the vicinity of the gap between the elements 21 and 22. A large potential difference is also applied across the gap between the lens element 22 and the dag coating 40. The strong focusing effect of the element 22dag gap produces a crossover or plane of minimum cross section in the vicinity of the deflection yoke 39, and a second image is formed in the plane of the target screen 12. The first image thus serves p as the object for the second lens formed by the element 22-dag gap. In this mode, the magnification of the entire lens arrangement is the product of the individual magnifications of the first and second lenses.

The axial position of the intermediate image is varied by changing the voltages on the lens elements. The magnification of the intermediate image is proportional to the element 16-21 lens to intermediate image distance divided by the element 16-21 lens to object (aperture) distance. The magnification of the final or screen image is proportional to the element 22-dag lens to final image distance divided by the element 22-dag lens to intermediate image distance. By shifting the position of the intermediate image, the magnification ratios may be varied.

By way of specific example, orders of magnification are set forth below for corresponding voltages on the elements 21 and 22. Such magnifications may be achieved while maintaining a sharp and bright image on the target screen. For the following magnifications and voltages, the accelerating potential 28 may be 4000 volts, the focus potential 25 may be 250 volts, and the anode potential With a voltage of volts on the element 21 and a voltage of -76 volts on the element 22, the lens arrangement 19 will act as an electron mirror, reflecting the electron beam. Thus, sufficiently negative voltages on both elements may be utilized to blank off the beam.

It is clear from the table above that the alternate or cascade mode of the lens arrangement 19 can be used to extend the range of magnification of the arrangement 19. In the first described mode of operation, wherein a single image is formed and the effective center of the lens is shifted along the tube axis, magnifications of about 3.6 to 23 are obtained. In the alternate or cascade mode, wherein two images are formed and the intermediate image is shifted along the tube axis, magnifications of about 26 to 37 are obtained.

In the embodiment of FIGURE 1, the lens formed by the gaps between the elements 15, 31 and 14 also provides some magnication. The image formed in the plane of the aperture plate 33 by lens action of such gaps is magnified to about 1.5 times the size of the corresponding character shaped aperture in the matrix plate 17. The overall magnification from matrix 17 to screen 12 is then variable from about 5.4 (first mode) to 55 (alternate mode).

The alternate embodiment of FIGURE 3 can also be operated in the cascade mode. In this mode, an intermediate image is formed within the element 58. Since the embodiment of FIGURE 3 has more elements, more gaps, and greater axial length, the range of magnifications obtainable using both modes is greater than the range obtainable with the embodiment of FIGURE l using both modes. The greatest range of magnifications and greatest flexibility in obtaining intermediate magnification is achieved by operating all three lens elements 56, 57, and S8 at selectively variable potentials. The range of magnifications can be further increased by using more than three lens elements and by changing the axial lengths of the elements.

It may therefore be seen that the invention provides an improved cathode ray tube of the character generating type for displaying character information on a target screen. Character image size on a target screen is variable while maintaining a sharply defined and bright image.

It is well known that electron beams can be focused by either electric fields or magnetic fields. While the particular embodiments described herein utilize electric fields, it is obvious that analogous structures utilizing magnetic fields can be designed, and will produce similar results.

Various modifications of the invention other than those shown and described herein will be apparent to persons skilled in the art from the foregoing description and accompanying drawings. Such modifications are intended to fall within the scope of the appended claims.

What is claimed is:

1. A display device comprising an evacuated envelope, an electron beam source, an electron responsive target, means for accelerating the electron beam along a path to impinge on said target, means for shaping the cross section of the electron beam into a predetermined configuration, at least three conductive lens elements disposed between said shaping means and said target for producing an image of the predetermined configuration of said beam on said target, said lens elements being spaced at intervals along a portion of the electron beam path so that adjacent lens elements are not in contact, thereby defining at least two non-conductive gaps, a field which focuses said beam being provided at each gap when a potential difference exists across each gap, and means for selectively varying the electric signals applied to said lens elements to vary the potential difference therebetween and thereby Vary the size of the image on said targe while maintaining the image sharpness.

2. A display device according to claim 1 wherein said beam shaping means include an electron opaque member defining a plurality of apertures each corresponding to one of a plurality of desired configurations of the beam cross section.

3. A display device according to claim 2 wherein said electron beam source provides a beam of sufficient size as to pass simultaneously through a plurality of said shaping apertures in said opaque member, thereby shaping the electron beam emitted by said source into a plurality of shaped beams, said display device further including electron beam defiection means disposed along the electron beam path intermediate said shaping means and said non-conductive gaps, said deflection means producing defiection of all of the plurality of shaped beams in response to selection signals applied thereto, an electron opaque selection member disposed along the electron beam path intermediate said defiection means and said electron lenses, said selection member defining a single selection aperture of a size sufficient to pass only one of said plurality of shaped beams, such one beam being selected by said deection means, said non-conductive gaps providing potential discontinuities to produce a sharp image of the beam at said single aperture on said target.

4. A display device according to claim 3 wherein a further plurality of lens elements forming at least one non-conductive gap are disposed along the electron path between said beam shaping member and said selection member for producing an image of said beam shaping member proximate the plane of said single aperture.

5. A display device according to claim 1 wherein said conductive elements comprise hollow members which are positioned so that the electron beam passes therethrough and wherein said field is an electric field.

. 6. A display device according to claim 1 wherein said non-conductive gaps are positioned immediately in succession.

7. A display device according to claim 1 wherein at least a portion of said accelerating means is disposed between said non-conductive gaps.

8. A display device according to claim 1 wherein said lens elements are of a configuration to produce an intermediate image in a plane transverse to the electron path and intermediate said nonconductive gaps.

9. A cathode ray tube comprising an evacuated envelope, an electron beam source, an electron responsive target, means for shaping the cross section of the electron beam produced by said source into a predetermined pattern, an electron lens along the electron path between said shaping means and said target for producing an image of said predetermined pattern on said target, said electron lens including at least two axially spaced potential discontinuities disposed along said beam path, said potential discontinuities being controlla-ble in magnitude by applying electric signals to said electron lens to thereby control the size of the image on said target while maintaining the sharpness of said image.

10. A display device comprising an evacuated envelope, an electron beam source, an electron responsive target, means for accelerating the electron beam along a path to impinge on said target, means for shaping the cross section of the electron beam into a predetermined configuration, a plurality of lens elements disposed between said shaping means and said target for producing an image of the predetermined configuration of said beam on said target, said lens elements defining at least two focusing fields disposed along successive portions of the electron beam path, said fields each having a focusing effect which varies in accordance with the magnitude of said fields between respective upper and lower levels, said focusing fields producing an image of a first size when said first field is at its upper level of magnitude and said second field is at its lower level of magnitude, and producing an image of a second size when said first field is at its lower level of magnitude and said second field is at its upper level of magnitude, and means for selectively varying the electric signals applied to said lens elements to vary the magnitude of each of said focusing fields between its respective upper and lower levels.

References Cited UNITED STATES PATENTS 2,383,751 8/1945 Spangenberg 315-14 2,902,623 9/1959 Knechtli 315-15 X 3,286,114 ll/1966 Schlesinger 315-15 3,421,044 1/1969 Murdock et al 315-14 RODNEY D. BENNETT, 1R., Primary Examiner HERBERT C. WAMSLEY, Assistant Examiner Us. C1. XR. 313-83; 315-15 

